It is known that sewage sludge is inevitably produced as a waste product during the treatment of water: each individual produces on average approximately 20 kilos per annum thereof, which represents, for Europe, more than 10 million tons per annum, expressed on a dry basis, i.e. 50 million tons of raw sludge when reference is made to “wet” sludge, which is composed of 20% of dry mailer and 80% of water.
Currently, there are several possible end uses for this sludge, in particular: disposal on a landfill site, incineration and land application: in each of these cases, the drying of the sludge turns out to be an essential stage which makes it possible to reduce by a factor of 4 the volumes to be stored, transported and disposed of. However, due to numerous restrictions (in particular technical, health, regulatory, and the like), such routes for the discharge of sludge are increasingly, complex and thus increasingly expensive and they may even be challenged, indeed even banned, in places.
It is seen that this drying currently constitutes an expanding market and that it represents a significant part of the resulting cost of the processes for disposing of/enhancing in value the waste products represented by the biological sludge resulting from plants for the treatment of wastewater. This part increases as a proportion of the budget of the treatment plant as the capacity of the treatment plant decreases.
The majority of plants for the drying of sludge are “thermal drying” plants. They consume large amounts of energy (approximately 1000 kWh per of water evaporated), in particular fossil fuels, and they require the presence of qualified personnel and high capital costs. For this reason, these solutions according to the current state of the art are, economically speaking, poorly suited to small or medium capacity plants.
It has also been envisaged to dry the sludge by solar radiation, this technique exhibiting the advantage of using renewable energy and a simple construction technology coming under the notion of greenhouses of horticultural type. In order to improve the drying performance of such solar plants, the greenhouses are generally equipped with means which provide for the turning over of the sludge to be dried, indeed even its progression along the drying plant. Thus, the product to be dried always exhibits a wet surface in contact with the air, thus preventing the formation of a “crust” at the surface of the bed of sludge and making it possible to improve the efficiency of evaporation of the water during the drying treatment.
The solar dryer arrangements known currently provide for the extraction of the water vapor resulting from the gradual drying of the sludge by known phenomena of natural convection induced by the differences in density of the air in the greenhouse (these differences being due to the temperature and humidity gradients); solar dryers of this type are sometimes equipped with fans in order to provide for forced circulation and forced replacement of the gaseous atmosphere of the dryer (forced convection).
In the known plants, these fans are situated in the top part of the greenhouse (that is to say, “in the roofing”) and they are proportioned so as to provide a certain level of renewal of the total volume of the air present in the greenhouse. For this reason, the cost of the “forced ventilation” element represents between 25 and 50% of the electricity consumption related to the operation of the dryer. In point of fact, optimization studies which have been carried out show that it is the renewal of the layer of air in immediate contact with the bed of sludge to be dried which has the greatest repercussions on the efficiency of the evaporation process.
The proportioning of a solar drying device is related to the weather aspects of the site where this device is installed. Furthermore, the operability of the drying equipment is extremely limited during the night and during winter: the annual availability, which is essentially diurnal, not exceeding 30% of the time. In addition, it is usual to regard, in temperate regions, 70% of the annual amount of water present in the sludge being evaporated during the hottest 3 months of the year.
These various requirements make it necessary to significantly exaggerate the size of the plants by providing large surface areas to dry relatively low amounts of sludge. Depending on the regions where the plant is installed, on the one hand, and on the degree of optimization of the processes employed, on the other hand, the surface area put into a solar dryer oscillates between 0.3 and 1 m2 of greenhouse per tonne of sludge to be treated and per year (initial solids content of 25% and final solids content of 75%).
The operation of the solar drying equipment is advantageously under the control of the variations in one or more parameters, such as, in particular: the solar radiation measured, the temperature of the air or the humidity of the air inside and outside the greenhouse.
The system is thus activated according to the drying capability of the air, without taking into account the water vapor partial pressure at the surface of the sludge.
Another disadvantage of the solutions according to the prior art to this solar drying is that the feeding of the sludge, at one of the ends of the dryer, is carried out using a mobile charging appliance of the “tracked tractor” type equipped with a mobile bucket. The sludge is deposited in relatively even piles at the inlet of the greenhouse, without the surface of the sludge deposit being truly optimized and integrated with the drying zone.